Supporters of pro-West challenger Viktor Yush- chenko, waving placards that read "Today or Never," pledged to camp out in the capital city's freezing streets and conduct an open-ended "campaign of civil disobedience and nonviolent struggle for the recognition of the election's true result."

Yushchenko declared he had won a "convincing victory" and called upon the world to support him.

But Mr. Yanukovich, defying the mood in the streets, reportedly took the presidential oath of office Tuesday night.

Though the Ukrainian Rada, or parliament, lacks the legal power to overturn the official vote count, any declaration it makes in days ahead would carry significant moral force.

"At the moment, the Rada is the most legitimate institution in the country, and whatever it decides will have a big impact on how events unfold," says Vyacheslav Nikonov, chairman of Politika, an independent think tank in Moscow.

Nearly final results gave Yanukovich a 3 percent margin over his rival. But at least one exit poll suggested Yushchenko won with an 11 percent margin. Western monitors from dozens of countries have said the voting fell far short of international standards. And the US warned that it might sharply cut back a $143 million aid package. Some action will definitely be taken if "in the final analysis, this election [proves] to be fundamentally flawed and tarnished," State Department spokesman Adam Ereli said Monday.

Ukrainian authorities announced they would deal firmly with any "disorders," while some Russian media reported Ukrainian armored forces and troops on the move toward Kiev. Speaking on TV Monday night, his desk draped with a Ukrainian flag, Yanukovich warned that a minority of Western-backed "radicals" were trying to reenact Georgia's "Rose Revolution" - by coincidence, exactly one year ago - in which massive and sustained street protests compelled President Eduard Shevardnadze to resign following a rigged election. "I categorically will not accept the actions of certain politicians who are now calling people to the barricades," Yanukovich said. "This small group of radicals has taken upon itself the goal of splitting Ukraine."

Ukrainian society, traditionally split between the nationalist west and heavily-Russified east, has been deeply stressed by a vitriolic campaign that pitted Yushchenko, a pro-NATO market reformer, against the Kremlin-backed Yanukovich. Tuesday, some 200,000 opposition supporters flooded the streets of Kiev, as did hundreds of thousands in the western cities of Lvov, Ternopol, Vinitsa, and Ivano-Frankovsk. Those cities have recognized a Yushchenko win, a development that highlights the constitutional peril Ukraine could face.

At risk is Ukraine's fragile, 13-year-old independence. Western Ukrainians, who have spent much of their history honing a strong sense of national identity under Polish and Austro-Hungarian rule, strongly favor Yushchenko's plans to steer Ukraine into NATO and the European Union as rapidly as possible. But half the country's 48 million people live in the industrial eastern zones, which were part of Russia and the USSR for more than 300 years. Many speak no Ukrainian and identify most closely with the Russian population just across the recently established border.

Yanukovich's pledge to make Russian the country's second official language, to allow dual Ukrainian-Russian citizenship, and to join a Kremlin-sponsored free-trade union with Russia, resonates strongly in Ukraine's eastern reaches.

"Ukrainians in the west may be passionate nationalists and Yushchenko supporters, but they are no less passionate in the east about their preference for Yanukovich," says Mr. Nikonov. He says that if the crisis escalates, coal miners and industrial workers from eastern zones could flood Kiev to offset Yushchenko's current advantage on the streets.

Russia may not remain on the sidelines if events spin out of control. During the election, President Vladimir Putin made two thinly veiled swings through Ukraine to back Yanukovich. And the Kremlin is unlikely to accept passively a Yushchenko win, which spells rejection of economic integration with Russia and a fast-track to NATO. Mr. Putin was the first world leader to congratulate Yanukovich on his victory.

"Russia aspires to become the nucleus of a union of post-Soviet states, and if we manage to keep Ukraine, there is a hope this can work," says Georgy Satarov, a former Kremlin adviser. "But if Ukraine leaves, there is no hope and Russia will have to face the West alone."